pinctrl: renesas: r8a7791: Use shorthands for reserved fields

Replace the full descriptions of reserved register fields by shorthands
with a negative field width, and merge adjacent reserved fields.

This reduces kernel size by 349 bytes.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/972808be595fd742afc6b7fc89751ca4788d6f62.1649865241.git.geert+renesas@glider.be